It has long been recognized that glass having a high refractive index (n.sub.D) and low density would be especially desirable in ophthalmic lenses requiring high corrections and in vehicles where it is very important to secure minimum weight. Hence, a low density glass is advantageous either for alleviating the weight for a person wearing a pair of correcting lenses, or for optimizing the useful load:power relation of the vehicle.
The low density, very high refractive index glasses presently known exhibit high dispersion or, expressed differently, have a low Abbe number (.nu.), i.e., frequently less than 30, with the consequence of iridescence appearing at the sides of the lenses.